Existing discrete semiconductor devices that handle large currents (such as transient voltage surge devices, Schottky diodes, bipolar transistors and vertical MOS devices) generally include electrical contacts on the top and the back of a semiconductor substrate. In such devices, the main current flows between the contact on the top and the contact on the back of the substrate. Accordingly, the current is generally distributed evenly over the whole bulk of the substrate.
When surface-mount device (SMD) packages were introduced, the problem of connecting the top contact to the same plane as the back contact was solved by using bond wires or clips.
With the further trend to miniaturisation, smaller packages are needed. In devices that handle large currents, the conductivity of silicon or other semiconductor materials is limited and accordingly it is generally not possible to shrink the size of the substrate. Because of this, it is necessary to maximise the ratio of volume of the substrate to the volume of the package.
Chip scale packages (CSPs), especially flip chip packages, can provide having a relatively large substrate volume with a relatively small package size. In some CSPs, nearly 100% of the package volume is silicon.
In CSPs, the contacts of the device are located on a common surface of the substrate. An example of such a device 100 is shown in FIG. 1. The device 100 includes a semi-conductor substrate 112 having a major surface 102. A first contact 104 and a second contact 106 are provided on the major surface 102. In use, the substrate 112 can be mounted with the major surface 102 facing downwards, with the contacts 104 and 106 soldered to a carrier (for example, a printed circuit board (PCB)).
A disadvantage of this approach is that the current flow within the substrate 112 is lateral (this is illustrated by the arrows in FIG. 1), which can lead to current crowding and local heating, which can greatly reduce the robustness of the device. In particular, and as illustrated by the arrows showing the current flow in FIG. 1, the current distribution within the substrate 112 is generally uneven, with higher current densities being present at the edges of the contacts 104 and 106 that are closest together. The uneven current distribution, current crowding and associated local heating can severely affect the current handling capabilities of such a device, particularly where it is intended that the device should handle larger currents. Accordingly, a device of the kind shown in FIG. 1 may not be suitable for certain high current applications.